1-Hexene is an important commercial product. In addition to its use as a specific chemical, it is also extensively used in polymerisation processes either as a monomer or co-monomer. This invention defines a catalyst system that facilitates the production of 1-hexene from ethylene in high selectivity, while avoiding the co-production of significant quantities of other higher oligomers and polyethylene.
In this regard, it is known from prior art (U.S. Pat. No. 6,184,428) that a nickel catalyst comprising a chelating ligand, preferably 2-diphenyl phosphino benzoic acid (DPPBA), a nickel precursor, preferably NiCl2.6H2O, and a catalyst activator, preferably sodium tetraphenylborate, catalyses the oligomerisation of ethylene to yield a mixture of linear olefins containing 1-hexene. The selectivity towards the linear C6 α-olefin is claimed to be 33%. Similarly the Shell Higher Olefins Process (SHOP process, U.S. Pat. Nos. 3,676,523 and 3,635,937) using a similar catalyst system is reported to yield 11 mass % 1-hexene in its product mixture (Chem Systems PERP reports 90-1, 93-6 and 94/95S12).
Ziegler-type technologies based on trialkylaluminium catalysts, independently developed by Gulf Oil Chemicals Company (Chevron, e.g. DE patent 1,443,927) and Ethyl Corporation (BP/Amoco, e.g. U.S. Pat. No. 3,906,053), are also commercially used to oligomerise ethylene to mixtures of olefins that reportedly contain 14-25 mass % 1-hexene (Chem Systems PERP reports 90-1, 93-6, and 94/95S12).
The selective trimerisation of ethylene to 1-hexene via transition metal catalysis has been extensively studied and patented. Some of these trimerisation catalysts are capable of trimerising longer chain olefins. This is an important feature, since the trimeric products derived from longer chain olefins could be utilised as synthetic lubricants (e.g. polyalphaolefins/PAOs), as well as in various other applications such as components of drilling muds and as feedstock to prepare detergents and plasticisers. Most of the known catalysts for selective ethylene trimerisation are chromium-based. Recently, chromium-based trimerisation catalyst systems, containing heteroatomic ligands with both phosphorus and nitrogen heteroatoms (WO 03/053891) as well as sulphur and nitrogen heteroatoms (WO 03/053890), have been developed by the applicant. These ligands include a spacer of at least one carbon atom between the heteroatoms to allow true tridentate coordination with the chromium. Tridentate coordination complexes are generally believed to be more selective towards 1-hexene than bidentate complexes. An example of such a heteroatomic ligand for ethylene trimerisation is bis-(2-diethylphosphino-ethyl)-amine. Although the catalyst system containing this ligand is extremely selective towards 1-hexene (with overall 1-hexene selectivity exceeding 96 mass %), it exhibits only moderate catalyst activities.
Another example of such a heteroatomic ligand with both phosphorus and nitrogen heteroatoms for ethylene trimerisation is (o-methoxyphenyl)2PN(Me)P(o-methoxyphenyl)2 as described in WO 02/04119. This patent application discloses the use of ligands described by the following general formula: (R1)(R2)X—Y—X(R3)(R4) wherein X is phosphorus, arsenic or antimony; Y is a linking group such as —N(R5)— and R1, R2, R3 and R4 are each independently hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl groups, at least one of which has a polar substituent which is not a phosphane, arsane or stibane group. The ethylene trimerisation catalyst system containing (o-methoxyphenyl)2PN(Me)P(o-methoxyphenyl)2 is somewhat less selective (claimed selectivities towards 1-hexene range between 75 and 91.5 mass %) than the system described in WO 03/053891, but it is more active. Thus, based on the above description, an essential attribute of the ligands disclosed in this patent application, is that at least one of the R1, R2, R3 and R4 groups must have a polar, or electron donating, substituent. Open literature shows that the use of (o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2, a compound without any such polar substituents on at least one of R1, R2, R3 and R4, as a ligand under catalytic conditions resulted in no catalytic activity towards 1-hexene (Anthea Carter et al., Chem. Commun., 2002, 858-859). The coordinating phosphorus heteroatoms in the above-mentioned ligand are spaced apart by one nitrogen atom. It is believed that the nitrogen atom does not coordinate with the chromium (at least in the absence of an activator) and that without any further electron donating atoms on the ligand, it is a bidentate system. It is further believed that any polar, or electron donating substituents especially in the ortho-position of the phenyl groups (R1, R2, R3 and R4) facilitate the formation of a tridentate system. This feature is reiterated in Chem. Commun., 2002, 858-859 by stating; “This has led us to hypothesise that the potential for ortho-methoxy groups to act as pendent donors and increase the coordinative saturation of the chromium centre is an important factor.”
The applicant has now shown that, contrary to the findings of Carter et al., excellent ethylene trimerisation activities and selectivities are indeed possible using inexpensive PNP ligands containing non-polar substituents on the ortho positions of the phenyl rings attached to the phosphorus. Higher overall selectivities are in fact achievable when using these ligand systems compared to ligands in which the ortho position has a polar substituent as was reported by Carter et al.